The most significant environmental impact of pH involves synergistic
effects. Synergy involves the combination of two or more substances
which produce effects greater than their sum.

This process is important in surface waters. Runoff from agricultural,
domestic, and industrial areas may contain iron, aluminum, ammonia, mercury or
other elements. The pH of the water will determine the toxic effects,
if any, of these substances. For example, 4 mg/l of iron would not present
a toxic effect at a pH of 4.8. However, as little as 0.9 mg/l of iron at a
pH of 5.5 can cause fish to die.

Synergy has special significance when considering water and wastewater
treatment. The steps involved in water and wastewater treatment require
specific pH levels. In order for coagulation (a treatment process) to
occur, pH and alkalinity must fall within a limited range. Chlorination, a
disinfecting process for drinking water, requires a pH range that is
temperature dependent.